In recent years, a flat panel type display apparatus (personal computer), represented by a liquid crystal color display device, has mainly been placed into portable utilization along with weight reduction thereof. Under such a situation, there exists a requirement for high-definition display of characters and color image display using a smaller screen.
For the purpose of achieving character display with high visibility in the case of the display of highly fine characters, Japanese Patent Laid-Open No. 2002-91369 (patent document 1) discloses a method in which, for example, in a color liquid crystal display device designed to make display of one pixel through the use of R (red), G (green) and B (blue) rectangular display elements, a character image which is an object of display is displayed in a state where each of the rectangular display elements is associated with one or more pixels.
The method disclosed in this patent document 1 first acquires a two-valued character image (binary character image) with triple size on the basis of font data in a character formation process using a rasterizer. This triple-size binary character image is mapped in a coordinate system associated with each rectangular display element and each pixel is then gradated through smoothing on this coordinate system so as to reduce the jaggy (notched portion) at character edge portions and a character image is displayed on each rectangular display element in a state associated with three pixels.
In general, in a case in which character display is made in a portable electronic device such as a portable telephone or PDA (Personal Digital Assistants), it is said that a character size of approximately 3 mm is optimum. Moreover, now, in a highest-definition liquid crystal display (liquid crystal panel) generally put on the market, the screen resolution is approximately 180 dpi (dot per inch). In a case in which character display is made on a liquid crystal panel having a screen resolution of approximately 180 dpi through the use of the method of tripling a character image size, which is disclosed in the above-mentioned patent document 1, the resolution level becomes approximately 500 dpi, which is equivalent to the display of a character image of approximately 3 mm with a resolution of approximately 60 dots×60 dots.
However, in the case of the above-mentioned conventional method, distortion can occur in a character image due to quantization error when a binary character image is mapped in each rectangular display element.
In general, a font (printing font) developed for printing is created using a mesh exceeding 1000 to 10000 dpi. In addition, for example, for expressing a character image with a size of 3 mm accurately through the use of such a printing font, there is a need to use a dot of approximately 120 to 1200 dpi per character. However, in the case of the liquid crystal display commonly put on the market as mentioned above, the resolution is lacking in accurate regeneration of the printing font, which causes a dislocation of stroke connection position and a distortion in a direction of the stroke width to occur when a character image is displayed through the use of the aforesaid conventional method, which can degrade the character quality.
For example, in the case of displaying complicated characters such as Japanese characters, the character stroke width (line width) and the spacing between lines constituting a character can become approximately one dot. Moreover, for the formation of a binary character image, depending upon the accuracy of the character formation process, the stroke position can be dislocated in units of one dot. If such a stroke position dislocation occurs, a distortion occurs particularly at a connection position between lines constituting a character, which introduces a possible striking degradation of the character quality.
FIGS. 19A and 19B are illustrations for explaining a distortion of a character in a conventional character image displaying method. FIG. 19A is an illustration of an example of a character image having no distortion and FIG. 19B is an illustration of an example of a character image having a distortion. As shown in FIG. 19B, there is a case in which a distortion occurs at a position of connection between lines constituting the character.
In addition, when the character stroke width (line width) is approximately one dot, depending upon the accuracy of the binary character image formation process, the stroke width sometimes becomes 2 dots in some stroke directions. When the binary character image is projected onto a rectangular coordinate system for mapping it into a rectangular display element, a distortion can occur in the stroke width thereof in some stroke directions (see widths A and B in FIG. 19B).
FIGS. 20A and 20B are illustrations for explaining a distortion of a character in the case of the conventional character image displaying method. FIG. 20A is an illustration of an example of a character image in which no distortion occurs in a rectangular coordinate system before the projection, and FIG. 20B is an illustration of an example of a character image in which a distortion occurs when the character shown in FIG. 20A is projected onto a rectangular coordinate system, with it being shown at a resolution lower than the actual one for easy observation of a distortion generated state. The distortion occurs at the connection positions as shown in FIG. 20B (for example, right-hand oblique lines of a Japanese character signifying a “wood”, and other portions).
In general, an outline font (printing font) is made up of data describing a contour of a character and, on the basis of the information on this contour, a character outline is formed according to a character size needed and the pixels in the outline are filled with the black values (0), thereby producing a character image (glyph).
Although a degradation of a character image quality does not occur if the character outline information can be produced at a resolution equal in level to that at the design of the font describing the character outline, when the resolution at the production is low, the designed coordinate value does not always agree with the produced coordinate value depending upon the outline regeneration accuracy, which causes a distortion mentioned above to occur in a stroke.
The present invention has been developed in consideration of these problems, and it is an object of the invention to provide a display apparatus, display control apparatus, display method, display control program and computer-readable recording medium recording the same program, capable of reducing the quantization error for displaying a character with high visibility in the case of displaying a high-definition character.
[Patent Document 1] Japanese Patent Laid-Open No. 2002-91369